H2 Elimination Research Articles

Experimental and theoretical studies of pyrolysis of chrysophanol and its derivatives

Rhubarb has been used as a tobacco additive recently. In order to understand the smoke emissions of herbal cigarettes with rhubarb additive, the pyrolysis of major components of rhubarb: chrysophanol and its derivatives (emodin, rhein and aloe-emodin) have been studied by synchrotron VUV photoionization mass spectrometry (SVUV-PIMS) combined with molecular-beam sampling technique. Various pyrolysis products are observed due to the different substituent groups. The main pyrolysis products of chrysophanol come from the CO and H2O elimination reactions. The CO, CO2 and HCOOH elimination reactions are the primary pyrolysis processes for rhein. Besides, elimination of H2 from aloe-emodin and loss of OH radical from both emodin and rhein also occur in the pyrolysis process. Furthermore, theoretical calculations are used to deduce the decomposition pathways and determine the geometries of pyrolysis products. Our work shows that the combination of SVUV-PIMS measurements and theoretical calculations is powerful to study pyrolysis behavior of complex organic compounds.

DFT studies of the adsorption and dissociation of H2O on the Al13 cluster: origins of this reactivity and the mechanism for H2 release

A theoretical study of the chemisorption and dissociation pathways of water on the Al₁₃ cluster was performed using the hybrid density functional B3LYP method with the 6-311+G(d, p) basis set. The activation energies, reaction enthalpies, and Gibbs free energy of activation for the reaction were determined. Calculations revealed that the H₂O molecule is easily adsorbed onto the Al13 surface, forming adlayers. The dissociation of the first H₂O molecule from the bimolecular H₂O structure via the Grotthuss mechanism is the most kinetically favorable among the five potential pathways for O-H bond breaking. The elimination of H₂ in the reaction of an H₂O molecule with a hydrogen atom on the Al cluster via the Eley-Rideal mechanism has a lower activation barrier than the elimination of H₂ in the reaction of two adsorbed H atoms or the reaction of OH and H. Following the adsorption and dissociation of H₂O, the structure of Al₁₃ is distorted to varying degrees.

Bis(phosphine)boronium salts. Synthesis, structures and coordination chemistry

The synthesis of a range of bis(phosphine)boronium salts is reported [(R2HP)2BH2][X] (R = Ph, (t)Bu, Cy) in which the counter anion is also varied (X(-) = Br(-), [OTf](-), [BAr(F)4](-), Ar(F) = 3,5-(CF3)2C6H3). Characterization in the solid-state by X-ray diffraction suggests there are weak hydrogen bonds between the PH units of the boronium cation and the anion (X(-) = Br(-), [OTf](-)), while solution NMR spectroscopy also reveals hydrogen bonding occurs in the order [BAr(F)4](-) < [OTf](-) < Br(-). [(Ph2HP)2BH2][BAr(F)4] reacts with RhH(PPh3)3, by elimination of H2, forming [Rh(κ(1),η-PPh2BH2·PPh2H)(PPh3)2][BAr(F)4] which shows a β-B-agostic interaction from the resulting base stabilised phosphino-borane ligand. Alternatively such ligands can be assembled directly on the metal centre by reaction of in situ generated {Rh(PPh3)3}(+) and Ph2HP·BH3 to afford [Rh(κ(1),η-PPh2BH2·PPh3)(PPh3)2][BAr(F)4], which was characterised by X-ray crystallography. Addition of H3B·PPh2H to the well-defined 16-electron "T-shaped" complex [Rh(P(i)Bu3)2(PPh3)][BAr(F)4] (characterised by X-ray crystallography) formed of a mixture of base-stabilised phosphino borane ligated complexes [Rh(κ(1),η-PR2BH2·PR3)(PR3)2][BAr(F)4] (R = (i)Bu or Ph). These last observations may lend clues to the formation of bis(phosphine)boronium salts in the catalytic dehydrocoupling reaction of phosphine boranes as mediated by Rh(I) compounds.

Mechanism for H2 release from potential hydrogen storage materials of phosphine alane and phosphine borane in the presence or absence of alane or borane: a theoretical study

Electronic structure calculations have been employed to probe the reaction mechanisms of hydrogen release from phosphine alane and phosphine borane in the presence or absence of alane and borane. Geometries of stationary points are optimized at the MP2/aug-cc-pVDZ level. Then, the energy profiles are refined at the CCSD(T)/aug-cc-pVTZ level based on the MP2 optimized geometries. In both AlH3PH3 and BH3PH3 monomer, H2 elimination cannot be compared with the Al–P and B–P bonds dissociation. The theoretical results demonstrate that both AlH3 and BH3 can facilitate H2 dissociation from AlH3PH3. However, the H2 production process becomes more difficult once the strong adduct is formed. Similarly, AlH3 and BH3 can also take part in a catalytic process for H2 loss from BH3PH3 and induce a substantial reduction of the energy barrier for H2 release. Comparison with the reaction mechanisms of H2 elimination from BH3PH3 with AlH3 or BH3 as the catalyst shows that BH3 is a better catalyst than AlH3. Moreover, BH3PH3 is more favored as a potential hydrogen storage material than AlH3PH3 in the presence of AlH3 or BH3. So a catalyst is required as the generation of H2 from phosphorous systems proceeds with an energy barrier much higher than the X–P (X = Al and B) bond energy.

A theoretical study on the decomposition mechanisms of deprotonated glycolic acid

Theoretical calculations at B3LYP and CCSD(T) levels have been performed on the decomposition mechanisms of glycolate. Two new pathways for H2 elimination were identified. The first one proceeds through a concerted mechanism which involves the loss of a α-hydrogen along with the hydroxyl hydrogen. The second pathway involves the step-wise loss of the two α-hydrogen atoms. The previously proposed mechanism initiated by nucleophilic attack proved to be not involved in the H2 elimination pathway. Two previously proposed pathways for CO elimination from glycolate proceeds through nucleophlic attack of the carboxylic oxygen on the α-carbon and through hydrogen transfers were confirmed by our theoretical calculations. The pathway leading to the CH2O elimination from glycolate was found to be branched from the intermediate formed after the nucleophlic attack. The identified pathways and calculated energy barriers well rationalized the experimentally observed fragment ions and their relative peak intensities of mass spectrums.

Theoretical Study on the Ring-Opening of 1,3-Disilacyclobutane and H2 Elimination

The kinetics and thermochemistry of the decomposition pathways for 1,3-disilacyclobutane (1,3-DSCB) in the gas phase were studied using the second-order Møller-Plesset (MP2) perturbation theory and coupled cluster methods with single, double, and perturbative triple excitations (CCSD(T)). The reactions examined include 2 + 2 cycloreversion to form two silenes by either a concerted or a stepwise mechanism, 1,1-, 1,2-, and 1,3-H(2) elimination, and the ring-opening initiated by 1,2-H shift to form an open-chain 1,3-disilabut-1-ylidene, which undergoes further decomposition to produce two pairs of silene/silylene species. The structures of the transition states for the concerted and the stepwise 2 + 2 cycloreversion pathways are found to resemble closely those reported for the head-to-tail and head-to-head dimerization, respectively. Comparison of the activation barriers demonstrates unambiguously that the stepwise cycloreversion (ΔH(0)(‡) = 66.1 kcal/mol) is favored over the concerted one (ΔH(0)(‡) = 77.3 kcal/mol). A new pathway was established from the 1,4-diradical intermediate in the stepwise cycloreversion to form 1-silylmethylsilene via 1,3-H shift. The concerted 1,1-H(2) elimination is shown to have the lowest activation barrier of all H2 elimination reactions. Overall, the 1,2-H shift in 1,3-DSCB with concerted ring-opening to form 1,3-disilabut-1-ylidene is the most kinetically and thermodynamically favorable decomposition pathway, both at 0 and 298 K.

Reactivity of lanthanoid mono-cations with ammonia: A combined inductively coupled plasma mass spectrometry and computational investigation

The behavior of La+, Sm+, Eu+ and Gd+ with NH3(g) and ND3(g) was studied to understand gas phase chemical reactions used for separations in the reaction cell of a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS). For Ln+=La+ and Gd+, the primary reaction channel is the formation of the LnNH+ protonated nitride leading to H2 elimination. The LnNH(NH3)1–5+ ammonia complexes of the Ln protonated nitride are further generated. Sm+ and Eu+ are less reactive: the protonated nitride is not detected, and only small amounts of Ln(NH3)0–6+ are observed. Quantum chemical calculations at the DFT, MP2, CCSD(T) and CASPT2 levels of theory were employed to explore the potential energy surfaces. For the La+ and Gd+ ions of f-block elements, the reaction pathways are composed of three steps: first the formation of LnNH3+, then the isomerization to HLnNH2+, and finally the loss of H2 associated with the formation of an LnN triple bond in the final product LnNH+. On the other hand, the isomerization leading to triple bond formation with H2 loss did not proceed for Sm+ and Eu+ ions.

Mechanistic Studies on the Gas-Phase Dehydrogenation of Alkanes at Cyclometalated Platinum Complexes

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L-H)](+) (L=2,2'-bipyridine (bipy), 2-phenylpyridine (phpy), and 7,8-benzoquinoline (bq)) with linear and branched alkanes C(n)H(2n+2) (n=2-4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L-H)](+)/C(2)H(6), loss of C(2)H(4) dominates clearly over H(2) elimination; however, the mechanisms significantly differs for the reactions of the "rollover"-cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen-atom transfer from C(2)H(6) to [Pt(bipy-H)](+), followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)](+), for the phpy and bq complexes [Pt(L-H)](+), the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L-H)(H(2))](+) as the ionic product accounts for C(2)H(4) liberation. In the latter process, [Pt(L-H)(H(2))(C(2)H(4))](+) (that carries H(2) trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C(2)H(4) rather than H(2) is ejected. For both product-ion types, [Pt(H)(bipy)](+) and [Pt(L-H)(H(2))](+) (L=phpy, bq), H(2) loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy-H)](+) with the higher alkanes C(n)H(2n+2) (n=3, 4), H(2) elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C(3)H(5))(bipy)](+). In the reactions of [Pt(L-H)](+) (L=phpy, bq) with propane and n-butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of "rollover"-cyclometalated [Pt(bipy-H)](+) with C(n)H(2n+2) (n=2-4) less than 15 % of the generated product ions are formed by C-C bond-cleavage processes, this value is about 60 % for the reaction with neo-pentane. The result that C-C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2-elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1- and 1,3-elimination pathways are only of minor importance.

Computational Insight into the Mechanism of Selective Imine Formation from Alcohol and Amine Catalyzed by the Ruthenium(II)‐PNP Pincer Complex

AbstractWe have used density functional theory computations to investigate the mechanism of the reaction of an amine with a primary alcohol catalyzed by the ruthenium(II)‐PNP pincer complex [PNP = 2,6‐bis(di‐tert‐butylphosphanylmethyl)pyridine]; the reaction produces an imine as the major product. The catalytic cycle includes four stages: (stage I) alcohol dehydrogenation to aldehyde, (stage II) coupling of aldehyde with amine to form hemiaminal, (stage III) hemiaminal dehydration to give imine, and (stage IV) catalyst regeneration by means of H2 elimination of the trans ruthenium dihydride complex produced in stage I. The mechanism is similar to that for amide formation from amine and alcohol that was catalyzed by the RuII‐PNN pincer complex [PNN = 2‐(di‐tert‐butylphosphanylmethyl)‐6‐(diethylaminomethyl)pyridine], the only difference being in stage III. Alcohol dehydrogenation (stage I) occurs by a bifunctional double hydrogen transfer mechanism and alcohol can facilitate stages II and III. The selectivity of imine over ester is governed by stage II: the formation of hemiaminal by means of aldehydeamine coupling is kinetically much more favorable than the alternative aldehydealcohol coupling reaction that yields hemiacetal. Furthermore, the hemiaminal dehydration to give an imine is also kinetically more favorable than the hemiacetal dehydrogenation to give an ester. The selectivity of imine over amide is determined by stage III: the hemiaminal dehydration to give an imine is kinetically much more favorable than the hemiaminal dehydrogenation to give an amide. The essential difference between the RuII‐PNP‐catalyzed imine synthesis and the RuII‐PNN‐catalyzed amide formation is that the former prefers hemiaminal dehydration whereas the latter prefers hemiaminal dehydrogenation. In addition, water produced during hemiaminal dehydration can catalyze stages II and III more effectively than alcohol can. By contrast, the water‐catalyzed hemiaminal formation does not happen in the RuII‐PNN‐catalyzed synthesis of an amide because no water is produced in any stage of the reaction.

Density functional theory studies on the mechanism of activation of methane by homonuclear bimetallic Ni–Ni

The reaction mechanisms of methane catalyzed by homonuclear bimetallic Ni–Ni have been investigated theoretically in this paper. Activation of methane by homonuclear bimetallic Ni–Ni is proposed to proceed in two effective catalytic reaction routes, Route I and Route II. The homonuclear bimetallic Ni–Ni inserts in the C–H bond to form the ring-hydride and chain-hydride in Route I and Route II, respectively. In this study, we examine the isomerization reaction pathway for ring-hydride (Route I). Undergoing the isomerization reaction, the chain-product CH3Ni–Ni–H is formed. From the chain-hydride CH3Ni2–H, two kinds of distinguishable reaction paths have been considered including isomerization reaction and H2 elimination reactions in Route II. The H2 elimination reactions are proposed to proceed along three parallel reaction pathways with the activation of the second C–H bond on the potential energy surfaces (PESs). The pathway of rearranging to isomer in Route II is preferred to the others. In comparison with heteronuclear bimetallic NiM+ (M=Cu, Ag) and homonuclear bimetallic Pt2+, the dehydrogenation of methane induced by homonuclear bimetallic Ni–Ni should be more favored thermodynamically and should give more branching.

Competitive activation of C–H and C–F bonds in gas phase reaction of Ir+ with CH3F: A DFT study

The spin-forbidden reaction mechanism of Ir+ with CH3F to yield H2 or HF has been investigated using density functional theory (DFT) calculations. The overall H2 elimination reaction is calculated to be exothermic, by 36.3 kcal/mol. One pathway is found on three potential energy surfaces (PESs), whereas the HF-elimination reaction is exothermic by 32.8 kcal/mol, two reaction pathways are identified on the quintet and triplet surfaces, only one reaction pathway is identified on the singlet surface. The reaction mechanism can be explained by the simple donor–acceptor model. According to the revealed mechanism and the calculated potential energy surfaces, H2-elimination is energetically much more favorable than HF-elimination. These conclusions are consistent with the experimental observations.

Dissociative Addition of Water to a Main Group 13 Metal Cluster: Computational Study of the Reaction of Gallium Dimer with H2O

We have investigated the lowest triplet and singlet potential energy surfaces (PESs) for the reaction of Ga(2) dimer with water. Under thermal conditions, we predict formation of the triplet ground state addition complex Ga(2)···OH(2)((3)B(1)) involving Ga···O···Ga bridge interaction. At the coupled cluster CCSD(T)/AE (CCSD(T)/ECP) computational levels, Ga(2)···OH(2)((3)B(1)) is bound by 5.5 (5.7) kcal/mol with respect to the ground state reactants Ga(2)((3)Π(u)) + H(2)O. Identification of the addition complex is in agreement with the experimental evidence from matrix isolation infrared (IR) spectroscopy reported recently by Macrae and Downs. The located minimum energy crossing points (MECPs) between the triplet and singlet energy surfaces on the entrance channel of Ga(2) + H(2)O are not expected to be energetically accessible under the matrix conditions, consistent with the lack of occurrence of Ga(2) insertion into the O-H bond under such conditions. The computed energies and harmonic and anharmonic vibrational frequencies for the triplet and singlet Ga(2)(H)(OH) insertion isomers indicate the singlet double-bridged Ga(μ-H)(μ-OH)Ga isomer to be the most stable and support the experimental IR identification of this species. The energy barrier for elimination of H(2) from the second most stable singlet HGa(μ-OH)Ga insertion isomer found to be 13.9 (12.9) kcal/mol is also consistent with the available experimental data.

Methane Activation by MH+ (M = Os, Ir, and Pt) and Comparisons to the Congeners of MH+ (M = Fe, Co, Ni and Ru, Rh, Pd)

The mechanism of ligated-transition-metal- [MH(+) (M = Os, Ir, and Pt)] catalyzed methane activation has been computed at the B3LYP level of density functional theory. The B3LYP energies of important species on the potential energy surfaces were compared to CCSD(T) single-point energy calculations. Newer kinetic and dispersion-corrected methods such as M05-2X provide significantly better descriptions of the bonding interactions. The reactions take place more easily along the low-spin potential energy surface. The minimum-energy pathway proceeds as MH(+) + CH(4) → M(H)(2)(CH(3))(+) → TS → MH(CH(2))(H(2))(+) → MH(CH(2))(+) + H(2). The ground states are (5)Π, (4)Σ(-), and (1)Σ(+) for OsH(+), IrH(+), and PtH(+), respectively. The energy level differences of the reactants between the high- and low-spin states gradually become smaller from OsH(+) to PtH(+), being 30.66, 9.17, and 0.09 kcal/mol, respectively. The C-H bond can be readily activated by MH(+) (M = Os, Ir, and Pt) with a negligible barrier in the low-spin state; thus, OsH(+), IrH(+), and PtH(+) are likely to be excellent mediators for the activition of the C-H bond of methane. H(2) elimination is quite facile without barriers in the presence of excess reactants. The products of the reactions of MH(+) (M = Os, Ir, and Pt) + methane are all carbene complexes MH(CH(2))(+). The exothermicities of the reactions are 3.99, 15.66, and 12.14 kcal/mol, respectively. The results for MH(+) (M = Os, Ir, and Pt) are compared with those for the first- and second-row congeners, and the differences in behavior and mechanism are discussed.

Ni(PMe2NPh2)2](BF4)2 as an Electrocatalyst for H2 Production

A nickel(II) bis(diphosphine) complex, [Ni(PMe2NPh2)2](BF4)2 (PMe2NPh2 = 1,5-diphenyl-3,7-dimethyl-1,5-diaza-3,7-diphosphacyclooctane), has been synthesized and characterized. This complex, which contains pendant amines in the diphosphine ligand, is an electrocatalyst for hydrogen production by proton reduction. Using [(DMF)H]OTf as the acid, a turnover frequency of 1,540 s–1 was obtained with no added water, and a turnover frequency of 6,700 s–1 was found with 1.0 M water added. Thermochemical studies show that the hydride donor ability of [HNi(PMe2NPh2)2](BF4) is ΔG°H– = 54.0 kcal/mol, and we estimate a driving force for H2 elimination of 13.8 kcal/mol. [Ni(PMe2NPh2)2](BF4)2 is the fastest H2 production catalyst in the [Ni(PR2NR′2)2](BF4)2 family of complexes.

Bio-hydrogen production from cheese whey powder (CWP) solution: Comparison of thermophilic and mesophilic dark fermentations

Cheese whey powder (CWP) solution was used as the raw material for hydrogen gas production by mesophilic (35 °C) and thermophilic (55 °C) dark fermentations at constant initial total sugar and bacteria concentrations. Thermophilic fermentation yielded higher cumulative hydrogen formation (CHF = 171 mL), higher hydrogen yield (111 mL H2 g−1 total sugar), and higher hydrogen formation rate (3.46 mL H2 L−1 h−1) as compared to mesophilic fermentation. CHF in both cases were correlated with the Gompertz equation and the constants were determined. Despite the longer lag phase, thermophilic fermentation yielded higher specific H2 formation rate (2.10 mL H2 g−1cells h−1). Favorable results obtained in thermophilic fermentation were probably due to elimination of H2 consuming bacteria at high temperatures and selection of fast hydrogen gas producers.

Experimental Study of Reductive Elimination of H2 from Rhodium Hydride Species

A RhIII monohydride terpyridine complex undergoes reductive elimination of H2 in CH3CN to form a dinuclear RhII complex with a metal–metal bond. Kinetic studies have revealed that the conversion fr...

Thermal Ammonia Activation by Cationic Transition-Metal Hydrides of the First Row - Small but Mighty

The thermal reactions of cationic 3d transition-metal hydrides MH(+) (M=Sc-Zn, except V and Cu) with ammonia have been studied by gas-phase experiments and computational methods. There are three primary reaction channels: 1) H(2) elimination by N-H bond activation, 2) ligand exchange under the formation of M(NH(3))(+), and 3) proton transfer to yield NH(4)(+). Computational studies of these three reaction channels have been performed for the couples MH(+)/NH(3) (M=Sc-Zn) to elucidate mechanistic aspects and characteristic reaction patterns of the first row. For N-H activation, σ-bond metathesis was found to be operative.

Studies of a Series of [Ni(PR2NPh2)2(CH3CN)]2+ Complexes as Electrocatalysts for H2 Production: Substituent Variation at the Phosphorus Atom of the P2N2 Ligand

A series of [Ni(P(R)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2) complexes containing the cyclic diphosphine ligands [P(R)(2)N(Ph)(2) = 1,5-diaza-3,7-diphosphacyclooctane; R = benzyl (Bn), n-butyl (n-Bu), 2-phenylethyl (PE), 2,4,4-trimethylpentyl (TP), and cyclohexyl (Cy)] have been synthesized and characterized. X-ray diffraction studies reveal that the cations of [Ni(P(Bn)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2) and [Ni(P(n-Bu)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2) have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni(P(Bn)(2)N(Ph)(2))(2)] was also synthesized and characterized by X-ray diffraction studies and shown to have a distorted tetrahedral structure. These complexes, with the exception of [Ni(P(Cy)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2), all exhibit reversible electron transfer processes for both the Ni(II/I) and Ni(I/0) couples and are electrocatalysts for the production of H(2) in acidic acetonitrile solutions. The heterolytic cleavage of H(2) by [Ni(P(R)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2) complexes in the presence of p-anisidine or p-bromoaniline was used to determine the hydride donor abilities of the corresponding [HNi(P(R)(2)N(Ph)(2))(2)](BF(4)) complexes. However, for the catalysts with the most bulky R groups, the turnover frequencies do not parallel the driving force for elimination of H(2), suggesting that steric interactions between the alkyl substituents on phosphorus and the nitrogen atom of the pendant amines play an important role in determining the overall catalytic rate.

Facile Carbon–Carbon Bond Formation and Multiple Carbon–Hydrogen Bond Activations Promoted by Methylene-Bridged Iridium/Ruthenium Complexes

The methylene-bridged complex [IrRu(CO)4(μ-CH2)(dppm)2][BF4] (1; dppm = μ-Ph2PCH2PPh2) reacts with 1,1-dimethylallene and 1,2-butadiene, resulting in coupling of the cumulene and the bridging methylene group, yielding the complexes [IrRu(CO)4(κ1:κ1-Me2C═CCH2CH2)(dppm)2][BF4] (2) and [IrRu(CO)4(κ1:κ1-MeCH═CCH2CH2)(dppm)2][BF4] (3), respectively, in which the resulting hydrocarbyl fragment is chelating on iridium. Reaction of 1 with 1,1-difluoroallene initially yields the analogous insertion product [IrRu(CO)4(κ1:κ1-F2C═CCH2CH2)(dppm)2][BF4] (6), which slowly converts to [IrRu(CO)3(κ1:κ1-F2C═CCH2CH2CO)(dppm)2][BF4] (7) by carbonyl migratory insertion. Reaction of 1 with propadiene leads to the elimination of 1,3-butadiene and the formation of [IrRu(CO)4(dppm)2][BF4]. In the presence of excess 1,1-dimethylallene, compound 2 reacts further via multiple C–H bond activations to yield [IrRu(CO)3(κ1-C(CH2CH3)═C(CH3)2)(μ-κ1:η2-C≡CC(CH3)═CH2)(dppm)2][BF4] (8) with the elimination of H2, together with [IrRu(CO)3(κ1-...

Computational Study on the Catalytic Role of Pincer Ruthenium(II)-PNN Complex in Directly Synthesizing Amide from Alcohol and Amine: The Origin of Selectivity of Amide over Ester and Imine

Density functional theory calculations have been performed to investigate the mechanism of the reactions of amines with primary alcohols to produce amides, catalyzed by the pincer complex Ru(II)-PNN (PNN = 2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine). The results lead us to propose a catalytic cycle that includes four stages: (stage I) alcohol dehydrogenation to aldehyde, (stage II) coupling of aldehyde with amine to form hemiaminal, (stage III) hemiaminal dehydrogenation to amide, and (stage IV) catalyst regeneration via H2 elimination of the trans Ru dihydride complex produced in the two dehydrogenation stages. Both of the dehydrogenation reactions proceed via the bifunctional double hydrogen transfer mechanism rather than the β-H elimination mechanism. The selectivity of amide over ester is determined by the coupling stage in which the aldehyde∧amine coupling to give hemiaminal is more favorable than aldehyde∧alcohol coupling to give hemiacetal. The competition between dehydrogenatio...